Magnetostrictive devices, which generate strain through application of a magnetic field, are employed as magnetostrictive sensors, magnetostrictive vibrators, and similar devices for generating or detecting displacement with precision. Conventionally, RE-containing intermetallic compounds such as TbFe2, DyFe2, and SmFe2 are employed as magnetostrictive materials. However, these intermetallic compounds generate only minute displacement, and precise control of the generated displacement is difficult. Therefore, these compounds cannot be applied to magneto-displacement devices for controlling minute displacement.
Meanwhile, GGG (gallium gadolinium garnet) is known to be a magnetic refrigerant which may be applicable to magnetic refrigerators or similar devices. However, GGG has not yet been used in a commercial product, because of its poor refrigeration efficiency upon application of a weak magnetic field provided by a permanent magnet.
In recent years, an RE-containing alloy having an NaZn13 phase (hereinafter referred to as “NaZn13-type RE-containing alloy”) has been found to exhibit large magnetostrain and high magnetocaloric effect. By virtue of these properties, this type of alloy is thought to be a candidate magnetostrictive material, magnetic refrigerant, etc.
Specifically, La(FeaSi1-a)13 (0.84≦a≦0.88) is disclosed to exhibit a magnetostrain as large as about 0.4% at 200 K under ≧4T (see, for example, non-patent reference 1).
It is also disclosed that, when the above compound is transformed into La(FeaSi1−a)13Hb (0.84≦a≦0.88, 1.0≦b≦1.6) through hydrogen absorption or similar treatment, Curie temperature can be controlled and magnetocaloric effect can be maintained at a high level (see, for example, non-patent reference 2).
Conventionally, NaZn13-type RE-containing alloys such as La(FeaSi1−a)13 (0.84≦a≦0.88) have been produced by weighing alloy raw materials such as high-purity La, Fe, Si, etc., so as to attain a desired alloy composition and mixing; melting the mixture through arc melting; and heating the product for a considerably long period of time (e.g., at 1,050° C. for 1,000 hours) in order to remove an undesired phase (see, for example, non-patent reference 2).
In the conventional method for producing NaZn13-type RE-containing alloys, the long-term heat treatment step for removing an undesired phase lowers productivity and increases costs in the production of NaZn13-type RE-containing alloys, devices employing the alloys, and other products employing the alloys.
Among RE-containing alloys having an NaZn13 structure, an La—Fe—Si alloy has been found to exhibit magnetic phase transition concomitant with a large entropy change in accordance with a change in the external magnetic field and to have no temperature hysteresis in the magnetocaloric effect. By virtue of these properties, this type of alloy is considered a candidate magnetic refrigerant.
The magnetic phase transition temperature of the La—Fe—Si alloy can be controlled by absorbing hydrogen into the alloy, and the change in entropy does not decrease even when hydrogen is absorbed (see non-patent reference 2). Therefore, when the magnetic phase transition temperature of the alloy is controlled to approximately room temperature and a permanent magnet is employed to generate a magnetic field, the alloy can be used as a magnetic refrigerant which can work at about room temperature.
In addition, this type of alloy, which exhibits a large, isotropic volume change under application of an external magnetic field, is also considered a candidate magnetostrictive material (see non-patent reference 1).
A conventionally known method for producing an La—Fe—Si alloy having an NaZn13 structure includes arc-melting raw material metals (i.e., La, Fe, and Si), to thereby form an alloy ingot; heating the alloy ingot in an inert atmosphere at 1,000 to 1,200° C. for 240 hours to 1,000 hours, to thereby form a mother alloy; re-melting the mother alloy; atomizing the formed molten alloy in an atmosphere for cooling, to thereby produce spherical particles; and absorbing hydrogen into the particles, to thereby control the magnetic phase transition temperature to a predetermined level (see patent reference 1).
However, the aforementioned conventional method for producing an RE-containing alloy powder has a drawback. Namely, since the method includes a long-term heat treatment and two melting steps, production costs increase and oxygen content of the alloy increases, even though low-cost material is used.
[Non-Patent Reference 1]
Maya FUJITA and Kazuaki FUKAMICHI, Itinerant-Electron Metamagnetic La(FexSi1−x)13 Compounds, “Solid-State Physics,” Vol. 37, No. 6, (2002), p. 419-427
[Non-Patent Reference 2]
Maya FUJITA, Shun FUJIEDA, and Kazuaki FUKAMICHI, Large Magnetic Volume and Magnetocaloric Effect of Itinerant-Electron Metamagnetic La(FexSi1−x)13 Compounds, “Materia,” Vol. 41, No. 4, (2002), p. 269-275
[Patent Reference 1]
Specification of Japanese Patent Application Laid-Open (kokai) No. 2003-96547
[Problems to be Solved by the Invention]
The present invention has been conceived under such circumstances. Thus, an object of the present invention is to provide a technique capable of producing an NaZn13-type RE-containing alloy at high efficiency without performing a long-period heat treatment step. Another object of the invention is to provide an NaZn13-type RE-containing alloy produced through the technique. Still another object of the invention is to provide a magnetostrictive device fabricated from the NaZn13-type RE-containing alloy. Yet another object of the invention is to provide a magnetic refrigerant produced from the NaZn13-type RE-containing alloy.
In addition, an object of the present invention is to provide an RE-containing alloy powder which is easily pulverizable, is not too brittle, and can be produced at low cost, within a short time, and without increasing the oxygen content of the RE-containing alloy powder or a sintered product of the alloy which is employed as a magnetic refrigerant or a magnetostrictive material.
The present inventor has carried out extensive studies in order to solve the aforementioned problem, and has discovered the following method for producing an RE-containing alloy, an RE-containing alloy, a method for producing an RE-containing alloy powder, an RE-containing alloy powder, a method for producing a sintered RE-containing alloy, a sintered RE-containing alloy, a magnetostrictive device, and a magnetic refrigerant.
The present invention comprises the following items (1) to (22).
(1) A first method for producing a first RE-containing alloy represented by formula R(T1−xAx)13−y (wherein R represents at least one species selected from among La, Ce, Pr, Nd, Sm, Eu, Th, Dy, Ho, Tm, Yb, Gd, and Lu; T represents at least one species selected from among Fe, Co, Ni, Mn, Pt, and Pd; and A represents at least one species selected from among Al, As, Si, Ga, Ge, Mn, Sn, and Sb (0.05≦x≦0.2; and −1≦y≦1)) comprising a melting step of melting alloy raw materials at 1,200 to 1,800° C.; and a solidification step of rapidly quenching the molten metal produced through the above step, to thereby form the first RE-containing alloy, wherein the solidification step is performed at a cooling rate of 102 to 104° C./second, as measured at least within a range of the temperature of the molten metal to 900° C.
(2) The method for producing an RE-containing alloy according to (1), wherein, in the melting step, the alloy raw material is melted in an inert gas atmosphere at 0.1 to 0.2 MPa.
(3) A method for producing the first RE-containing alloy according to (1), wherein in the solidification step, the molten metal is rapid-quenched through any of strip casting, new centrifugal casting, and centrifugal casting.
(4) A method for producing the first RE-containing alloy according to (3), wherein the molten metal is rapidly quenched through strip casting in the solidification step, to obtain strips having a thickness of 0.1 to 2.0 mm.
(5) A second method for producing a second RE-containing alloy comprising a melting step and a solidification step for producing a first RE-containing alloy according to (1), and a heat treatment step of heating at 900 to 1,200° C. the first RE-containing alloy that is produced through the solidification step, to thereby form an NaZn13 phase.
(6) The second method for producing a second RE-containing alloy according to (5), wherein the NaZn13 phase is formed through the heat treatment step, which is performed for a period of from one minute to 200 hours.
(7) The second method for producing a second RE-containing alloy according to (6), wherein the heat treatment is performed at a temperature of 1080° C. to 1200° C. and for a period of from 3 to 42 hours.
(8) The first RE-containing alloy which is obtainable through the method of any one of (1) to (4).
Through the first methods of the present invention for producing a first RE-containing alloy (that is the above (1) to (4)), an RE-containing alloy suitably used for producing an NaZn13-type RE-containing alloys (i.e., a starting alloy for an NaZn13-type RE-containing alloy) are produced. NaZn13-type RE-containing alloys are produced through the second methods of the present invention for producing RE-containing alloys ((5) and (6)).
(9) A first RE-containing alloy, which is represented by formula R(T1−xAx)13−y (wherein R represents at least one species selected from among La, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Tm, Yb, Gd, and Lu; T represents at least one species selected from among Fe, Co, Ni, Mn, Pt, and Pd; and A represents at least one species selected from among Al, As, Si, Ga, Ge, Mn, Sn, and Sb (0.05≦x≦0.2; and −1≦y≦1)), and which comprises an R-rich phase, having a relatively high rare earth metal (R) content, and an R-poor phase, having a relatively low rare earth metal (R) content, wherein the R-rich phase and the R-poor phase are dispersed at a phase spacing of 0.01 to 100 μm.
In the present specification, the R-rich phase spacing, the R-poor phase spacing, and the size of each phase are evaluated by use of back-scattered electron images of the alloy observed under a scanning electron microscope.
In a back-scattered electron image of the alloy, a portion having a large average atomic weight is observed as a white image, whereas a portion having a small average atomic weight is observed as a black image. In other words, the R-rich phase is observed as a white image, and the R-poor phase, having a rare earth metal content lower than that of the R-rich phase, is observed as a gray image. In a specific procedure, a back-scattered electron image of an alloy sample is taken at an appropriate magnification as a rectangular image, and the image data are converted to two values corresponding to black and white by use of image processing software. Subsequently, a lateral segment and a longitudinal segment through the center, and two diagonals (total four segments) are drawn in the rectangular image. The total lengths of the white portions that intersect each segment are measured, and the four measured lengths are averaged, to thereby derive the size of the R-rich phase (i.e., equivalent to R-poor phase spacing). In a similar manner, the total length of the black portions that intersect each segment is measured, and the four measured lengths are averaged, to thereby derive the size of the R-poor phase (i.e., equivalent to R-rich phase spacing).
(10) A second RE-containing alloy, which is represented by formula R(T1−xAx)13−y (wherein R represents at least one species selected from among La, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Tm, Yb, Gd, and Lu; T represents at least one species selected from among Fe, Co, Ni, Mn, Pt, and Pd; and A represents at least one species selected from among Al, As, Si, Ga, Ge, Mn, Sn, and Sb (0.05≦x≦0.2; and −1≦y≦1)), wherein the alloy has an NaZn13 phase content of at least 90 vol. %.
(11) A magnetostrictive device provided from the second RE-containing alloy according to (10).
(12) A magnetic refrigerant provided from the second RE-containing alloy according to (10).
The magnetostrictive device and magnetic refrigerant of the present invention are characterized by being produced from the aforementioned second RE-containing alloy of the present invention (an NaZn13-type RE-containing alloy).
(13) An RE-containing alloy, which is represented by a compositional formula of RrTtAa (wherein R represents at least one rare earth element selected from among La, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Tm, Yb, Gd, and Lu; T collectively represents transition metal elements containing at least Fe atoms, a portion of the Fe atoms being optionally substituted by at least one species selected from among Co, Ni, Mn, Pt, and Pd; A represents at least one element selected from among Al, As, Si, Ga, Ge, Mn, Sn, and Sb; and r, t, and a have the following relationships: 5.0 at. %≦r≦6.8 at. %, 73.8 at. %≦t≦88.7 at. %, and 4.6 at. %≦a≦19.4 at. %) and having an alloy microstructure containing an NanZn13-type crystal structure in an amount of at least 85 mass % and α-Fe in an amount of 5-15 mass % inclusive.
(14) A method for producing an RE-containing alloy powder, comprising pulverizing, by mechanical means, the RE-containing alloy according to (13) to a powder having a mean particle size of 0.1 μm to 1.0 mm.
(15) An RE-containing alloy powder comprising an RE-containing alloy according to (13), which has a mean particle size of 0.1 μm to 1.0 mm.
(16) A magnetic refrigerant comprising the sintered RE-containing alloy powder according to (15), wherein the Curie temperature of the magnetic refrigerant has been controlled through absorption of hydrogen in the sintered RE-containing alloy.
(17) A method for producing a sintered RE-containing alloy, which comprises compacting an RE-containing alloy powder produced through a method for producing an RE-containing alloy powder as described in (14), and sintering the compact.
(18) The method for producing a sintered RE-containing alloy described in (17), wherein the sintering is performed at 1,200° C. to 1,400° C.
(19) The method for producing a sintered RE-containing alloy described in (17) or (18), wherein, after completion of sintering the RE-containing alloy powder, the sintered alloy is maintained in a hydrogen atmosphere at 200° C. to 300° C., to thereby absorb hydrogen into the sintered alloy.
(20) A sintered RE-containing alloy, which is formed by compacting the RE-containing alloy powder as recited in (15), and sintering the compact.
(21) A magnetostrictive material comprising the sintered RE-containing alloy as recited in (20), wherein the Curie temperature of the magnetostrictive material has been controlled through absorption of hydrogen into the sintered RE-containing alloy.
(22) A magnetic refrigerant comprising the sintered RE-containing alloy as recited in (20), wherein the Curie temperature of the magnetic refrigerant has been controlled through absorption of hydrogen into the sintered RE-containing alloy.
According to the present invention, there can be provided a technique capable of producing an NaZn13-type RE-containing alloy at high efficiency without performing long-term heat treatment; an NaZn13-type RE-containing alloy produced through the technique; and a magnetostrictive device and a magnetic refrigerant obtained from the NaZn13-type RE-containing alloy.
Further, according to the present invention, a magnetostrictive material and a magnetic refrigerant formed of an RE-containing alloy (e.g., La—Fe—Si) having an NaZn13 structure can be reliably produced at low cost, compared with conventional methods. The present invention contributes toward mass production of magnetic refrigerator and magnetostrictive devices.